The present invention relates to sound generating systems and in particular to speaker system for transforming electrical signals into audio sound over substantially the entire audio frequency spectrum.
Typically, speakers utilized in home stereo systems or as monitors in recording studios include several individual speakers extending into a housing and arranged on a speaker mounting panel for projecting audio through openings in the mounting panel. Such speaker systems generally operate by dividing the audio frequency spectrum into several segments and then providing a different speaker to transform the electrical signal components in each spectrum segment into an audio sound. Such systems require that a cross-over network be used to interconnect the speakers so that the spectrum of frequencies actuating one of the speakers is isolated from the spectrum of frequencies actuating each of the other speakers.
More specifically, most present speaker systems include a woofer which is actuated in response to the low frequency segment of the audio frequency spectrum; a midrange speaker which is actuated in response to the midrange frequency segment of the audio frequency spectrum; and a tweeter which is actuated in response to the high frequency segment of the audio frequency spectrum. As previously indicated, complex cross-over networks to separate the portions of the frequency spectrum which will actuate each speaker are then incorporated.
In the past, considerable effort has gone into perfecting the cross-over networks. However, such networks inevitably have limitations which significantly effect the quality of audio sound generated by the speakers both separately and collectively. For example, the speakers utilized may be so susceptible to interaction between each other that the cross-over network must be designed to have steep cross-over slopes. Such a requirement frequently results in severe spikes, peaks, ringing and phase shifts in the response of the speaker which would be considered unacceptable if observed even in the most rudimentary audio receiver or amplifier.
Cross-over networks also have the effect of causing a loss of efficiency which results in additional amplifier distortions or higher amplifier power requirements. They also have the effect of reducing the dampening factor that most amplifier designers design into their amplifiers to help reduce excessive cone motion of the woofer. This dampening factor also improves transient response of the woofer. However, this dampening factor tends to be reduced, and its attendant advantages eliminated, by the fact that the cross-over network in most speaker systems becomes the "window" that the amplifier is looking at rather than the speaker which the dampening factor can control. Indeed, many amplifiers exhibit severe cross-over notch distortion and instability as the result of having to "look into" a cross-over network with a combined inductive and resistive load. The result is a serious degradation in the clarity of the audio sound produced by the speaker system in response to electrical signals from the receiver or amplifier.
The utilization of a woofer and midrange speaker, which is the reason the above-described cross-over network is required, produces other inherent distortions in the audio sound generated. Specifically, a woofer, which may typically be in the range of about 12 to 15 inches in diameter, generally has a high mass and extreme cone excursion to enable it to move large quantities of air at the low frequency levels. However, that high mass and extreme cone excursion makes the typical woofer incapable of reproducing upper bass frequencies which occur simultaneously with sounds in the lower bass register. In addition to this dynamic response degradation and inefficiency, transient response in such woofers also suffers, particularly in acoustic suspension designs where movement of the speaker cone is impeded by the compressed volume of air within a tightly enclosed housing. The result is a severe loss of definition and dynamics at the high volume levels and a considerable change in apparent listening characteristics coupled with the loss of bass at the low listening levels.
In addition to the woofer, most speaker systems incorporate a midrange speaker, typically having a three to five inch diameter, to project the midrange band of frequencies in the audio frequency spectrum. However, the relatively small size of the typical midrange speaker is a paradox since it cannot move enough air in the vital midrange in which it is supposed to operate. Consequently, loud orchestral passages with full bass of rock music with hard driving guitar sounds are often reproduced with a severely limited midrange air motion totally out of proportion to either the bass or treble frequencies, thereby limiting reproduction of the original dynamics of the sound source. Furthermore, the inability of the midrange speaker to move the amount of air required at high audio volumes results in a quite strident and fatiguing sound to the listener's ear. In an effort to alleviate these problems, some systems utilize a midrange horn which increases efficiency and air loading. However, the midrange horn produces a very hollow middle register sound with increased stridency and harshness. It will also be appreciated that full control of the cone motion is desired to improve the midrange speaker's transient response. Such control would be provided utilizing the amplifier's signal dampening (dampening factor) and the back EMF forces produced by the motion of the components in the speaker system. However, in a typical speaker system the cross-over network in effect "absorbs" the speaker back EMF effects and the dampening effects from the amplifier. Such cross-over network "absorption" requires compensation to restore some measure of control over cone motion. Typically, this compensation is provided by designing the spider or suspension since experience has shown that excessive cone motion must be controlled at the suspension points of the speakers. However, such an approach to controlling cone motion also reduces transient response of the woofer and midrange speakers. Thus, the ideal speaker should not only have the necessary frequency response characteristics, but also excellent transient response and excellent dymanic response characteristics. However, in the typical speaker system with a cross-over network and a typically sized midrange speaker, the cross-over network automatically prevents modification of the speaker suspension to maximize transient response and dynamic response characteristics.
In addition to electrical signal separation between speakers in a speaker system, speaker system designers have also sought acoustic separation, particularly between the midrange speaker and the woofer. Such acoustic separation and isolation is desired to minimize distortion caused by the imposition of, e.g., the bass sound waves against the back of the midrange speaker cone which prevent the midrange speaker from moving as freely as it would otherwise move.
In some prior art systems, the desired acoustic separation and isolation has, to a degree, been achieved by physically isolating the midrange speaker in a box within the speaker system housing. The backwave, i.e., the sound projected from the back of the woofer into the housing around the woofer, may then be channeled through a ducted portion which may either be a hollow cylinder or may be a cylinder filled with an acoustic material, to provide a passageway for directing the backwave from the woofer out through the front of the speaker. In this manner, the backwave of the bass can be separated from the backwave of the midrange. However, even in systems where the backwave of the woofer is projected through a ducted port, the backwaves of the midrange speaker are characteristically suppressed. This backwave suppression is a consequence of the enclosure incorporated to obtain acoustic isolation and separation in the speaker system. It also automatically takes place since the cabinet is regarded almost entirely as a means of containing or at most augmenting the backwave response of the woofer in the bass frequency range. The present invention does not utilize the speaker enclosure as a bass frequency control mechanism. Rather the enclosure provides maximum utilization of the backwaves over a wide band of frequencies, specifically including the middle and upper middle frequencies. However, this usage, to be most effective, must make use of a relatively large total sound radiating surface. The typically sized midrange speaker, even if its backwave were to be utilized, cannot move enough air for its backwave motion to be of any significance. Thus, most speaker systems, at least in the middle and upper middle frequencies, as a result of not utilizing this backwave response over a wide band of frequencies, specifically including middle and upper middle frequencies, act to "rectify" the final audio sound characteristic. This suppression of the backwave audio response over these vital middle and upper middle frequencies results in a significant decrease in dynamic response and ambient characteristics because this backwave is not simply a phase shifted reproduction of the frontwave but is a complex wave motion which can contribute significantly to dynamic response and the naturally occurring ambient characteristics of the original sound source.